Conventional and commercially available melting systems include reverberatory, crucible, open hearth, cupola, electric induction, electric channel, electric arc, fuel assisted electric arc, conventional cupola, gas/oil fired vertical shaft furnaces with water-cooled grates and fuel-fired rotary drum furnaces. While all of these systems can and do operate to melt and superheat metal, there is room for improvement of each of these systems with respect to cost of installation, adaptability to a wide selection of fuels, cost of operation, cost of maintenance, flexibility of operation, equipment size, control of metallurgical properties, degree of potential pollution, installation cost of pollution controls and cost of operation of pollution controls.
Cupola melting units are efficient, low-cost melting devices especially in high ton-per-hour (over 50) units; however, they have a number of drawbacks. Using coke as the primary fuel, cupola operations are undesirably dependent on coke prices and availability. Because of the sulphur content of the coke, cupola melted irons can have excessive sulphur levels that are a problem in the production of ductile irons. Cupola off-gases are especially toxic and expensive to clean in order to meet governmental air quality standards for sulphur and particulate emissions. Cupolas are difficult to operate on an intermittent basis. When stopped, the delicate temperature equilibriums necessary for effective operation are destroyed and metallurgical control is made extremely difficult.
It is known to melt a charge in a vertical shaft furnace by feeding the charge into the top of the shaft furnace to form a column of charge in the furnace, and then melting the column from below with a flame. See, e.g., U.S. Pat. No. 5,224,985 to Kullik et al., U.S. Pat. No. 4,877,449 to Khinkis, U.S. Pat. No. 4,097,028 to Langhammer and U.S. Pat. No. 1,713,543 to Machlet.
U.S. Pat. No. 5,560,304 to Duchateau et al. discloses a furnace which employs a gas oxygen burner on a rotating barrel furnace; however this furnace is large and operates in a batch mode rather than continuously.
Vertical shaft furnaces with water-cooled grates operate successfully, but also have a number of drawbacks. The water-cooled pipes or grates have the potential for damage and leaking in such an environment. Water and water vapor are extremely detrimental to metallurgical properties of metal in the molten, superheated state of such a furnace. Further, the flame temperature and the metal temperature difference and therefore the efficiency of the system suffers as the operator attempts go above 2550.degree. F. Metal in the molten condition at such temperatures is also highly susceptible to undesired oxidation. Metal is then guided into some form of electric furnace for superheating to temperatures required for treatment, pouring, processing or casting. This method is inefficient from an energy perspective and further exposes the molten metal to oxidizing conditions. Silicon losses can go as high as 1% of the charge weight, a loss that can be very expensive as the lost silicon must be replaced. Oxidized irons also exhibit inferior metallurgical properties.
Electric melting, whether it be induction, induction channel or arc, is an efficient and effective way to process metal. Recovery of ingredients and additives is nearly 100%, as there is little oxidation since the metal charges are preheated primarily for removing water and for a small assist in melting efficiency. Arc furnaces can be fitted with fuel burners to assist in the melting down of the charge and are highly efficient melters. However, installation of dust collectors, which is required for electric furnaces, is a major expense. Further, installation of a large electric furnace is very costly. In terms of overall efficiency from a macroeconomic perspective, electric furnaces are not necessarily very efficient, since only 22% of the coal burned in a power plant is typically converted to electric power, which can then be used to power an electric furnace. The remainder is wasted heat and excessive production of CO.sub.2 gases.
Thus, there has been a need for more efficient furnaces for melting metals, which are less costly to install and maintain.
All references cited herein are incorporated herein by reference in their entireties.